ℹ️ This repository is part of my Refactoring catalog based on Fowler's book with the same title. Please see kaiosilveira/refactoring for more details.

---

Inline Class

Before	After
class Person { get officeAreaCode() { return this._telephoneNumber.areaCode; } get officeNumber() { return this._telephoneNumber.number; } } class TelephoneNumber { get areaCode() { return this._areaCode; } get number() { return this._number; } }	class Person { get officeAreaCode() { return this._officeAreaCode; } get officeNumber() { return this._officeNumber; } }

Inverse of: Extract class

As a result of the flexibility we have in healthy codebases, we often move things around to test new ideas, to better isolate responsibilities, and/or to reallocate behavior throughout our layers. Sometimes, though, we go too far. This refactoring helps in these cases where we want to merge two or more classes into a single unit.

Working example

Our starting point for this example is a Shipment class, which holds a reference to a TrackingInformation instance that holds its tracking data. We want to inline TrackingInformation, so all the tracking-related data is part of Shipment.

Supporting test suite

To make sure our refactoring steps don't break anything, we have the following supporting test suite:

describe('Shipment', () => {
  it('should provide a readable text of its tracking info', () => {
    const shipment = new Shipment();

    shipment.shippingCompany = 'DHL';
    shipment.trackingNumber = '1234567890';

    expect(shipment.trackingInfo).toEqual('DHL: 1234567890');
  });
});

It basically covers the most complex functionality in our system: printing the tracking info of a shipment, which delegates to the TrackingInformation.display method for this work.

Steps

To start things up, we introduce a delegate to shippingCompany at Shipment. This will help us moving the field later:

diff --git a/src/caller.js b/src/caller.js
@@ -5,7 +5,7 @@ const aShipment = new Shipment();
 const trackingInfo = new TrackingInformation();
 aShipment.trackingInformation = trackingInfo;

-aShipment.trackingInformation.shippingCompany = 'DHL';
+aShipment.shippingCompany = 'DHL';
 aShipment.trackingInformation.trackingNumber = '1234567890';

 console.log(aShipment.trackingInfo);

diff --git a/src/shipment/index.js b/src/shipment/index.js
@@ -10,4 +10,8 @@
export class Shipment {
   set trackingInformation(aTrackingInformation) {
     this._trackingInformation = aTrackingInformation;
   }
+
+  set shippingCompany(arg) {
+    this._trackingInformation.shippingCompany = arg;
+  }
 }

diff --git a/src/shipment/index.test.js b/src/shipment/index.test.js
@@ -7,7 +7,7 @@
describe('Shipment', () => {
     const trackingInfo = new TrackingInformation();
     shipment.trackingInformation = trackingInfo;

-    shipment.trackingInformation.shippingCompany = 'DHL';
+    shipment.shippingCompany = 'DHL';
     shipment.trackingInformation.trackingNumber = '1234567890';

     expect(shipment.trackingInfo).toEqual('DHL: 1234567890');

By the same token, we introduce a delegate to trackingNumber:

diff --git a/src/caller.js b/src/caller.js
@@ -6,6 +6,6 @@ const trackingInfo = new TrackingInformation();
 aShipment.trackingInformation = trackingInfo;

 aShipment.shippingCompany = 'DHL';
-aShipment.trackingInformation.trackingNumber = '1234567890';
+aShipment.trackingNumber = '1234567890';

 console.log(aShipment.trackingInfo);
diff --git a/src/shipment/index.js b/src/shipment/index.js
@@ -14,4 +14,8 @@
export class Shipment {
   set shippingCompany(arg) {
     this._trackingInformation.shippingCompany = arg;
   }
+
+  set trackingNumber(arg) {
+    this._trackingInformation.trackingNumber = arg;
+  }
 }

diff --git a/src/shipment/index.test.js b/src/shipment/index.test.js
@@ -8,7 +8,7 @@
describe('Shipment', () => {
     shipment.trackingInformation = trackingInfo;

     shipment.shippingCompany = 'DHL';
-    shipment.trackingInformation.trackingNumber = '1234567890';
+    shipment.trackingNumber = '1234567890';

     expect(shipment.trackingInfo).toEqual('DHL: 1234567890');
   });

Then, we can start moving things. We introduce a getter for shippingCompany at Shipment

diff --git a/src/shipment/index.js b/src/shipment/index.js
@@ -11,6 +11,10 @@
export class Shipment {
     this._trackingInformation = aTrackingInformation;
   }

+  get shippingCompany() {
+    return this._trackingInformation.shippingCompany;
+  }
+
   set shippingCompany(arg) {
     this._trackingInformation.shippingCompany = arg;
   }

...and a getter for trackingNumber:

diff --git a/src/shipment/index.js b/src/shipment/index.js
@@ -19,6 +19,10 @@
export class Shipment {
     this._trackingInformation.shippingCompany = arg;
   }

+  get trackingNumber() {
+    return this._trackingInformation.trackingNumber;
+  }
+
   set trackingNumber(arg) {
     this._trackingInformation.trackingNumber = arg;
   }

Finally, we can start inlining things. We first inline trackingInformation.display into Shipment.trackingInfo:

diff --git a/src/shipment/index.js b/src/shipment/index.js
@@ -1,6 +1,6 @@
 export class Shipment {
   get trackingInfo() {
-    return this._trackingInformation.display;
+    return `${this.shippingCompany}: ${this.trackingNumber}`;
   }

   get trackingInformation() {

And then we move the shippingCompany field:

diff --git a/src/shipment/index.js b/src/shipment/index.js
@@ -12,11 +12,11 @@
export class Shipment {
   }

   get shippingCompany() {
-    return this._trackingInformation.shippingCompany;
+    return this._shippingCompany;
   }

   set shippingCompany(arg) {
-    this._trackingInformation.shippingCompany = arg;
+    this._shippingCompany = arg;
   }

   get trackingNumber() {

Finally, we move the trackingNumber field:

diff --git a/src/shipment/index.js b/src/shipment/index.js
@@ -20,10 +20,10 @@
export class Shipment {
   }

   get trackingNumber() {
-    return this._trackingInformation.trackingNumber;
+    return this._trackingNumber;
   }

   set trackingNumber(arg) {
-    this._trackingInformation.trackingNumber = arg;
+    this._trackingNumber = arg;
   }
 }

And now we can stop using TrackingInformation:

diff --git a/src/caller.js b/src/caller.js
@@ -1,9 +1,6 @@
 import { Shipment } from './shipment/index.js';
-import { TrackingInformation } from './tracking-information/index.js';

 const aShipment = new Shipment();
-const trackingInfo = new TrackingInformation();
-aShipment.trackingInformation = trackingInfo;

 aShipment.shippingCompany = 'DHL';
 aShipment.trackingNumber = '1234567890';

diff --git a/src/shipment/index.js b/src/shipment/index.js
@@ -3,14 +3,6 @@
export class Shipment {
     return `${this.shippingCompany}: ${this.trackingNumber}`;
   }

-  get trackingInformation() {
-    return this._trackingInformation;
-  }
-
-  set trackingInformation(aTrackingInformation) {
-    this._trackingInformation = aTrackingInformation;
-  }
-
   get shippingCompany() {
     return this._shippingCompany;
   }

diff --git a/src/shipment/index.test.js b/src/shipment/index.test.js
@@ -1,11 +1,8 @@
 import { Shipment } from '.';
-import { TrackingInformation } from '../tracking-information';

 describe('Shipment', () => {
   it('should provide a readable text of its tracking info', () => {
     const shipment = new Shipment();
-    const trackingInfo = new TrackingInformation();
-    shipment.trackingInformation = trackingInfo;

     shipment.shippingCompany = 'DHL';
     shipment.trackingNumber = '1234567890';

To clean things up, we can now safely delete TrackingInformation:

diff --git a/src/tracking-information/index.js b/src/tracking-information/index.js
deleted file mode 100644
index d108113..0000000
--- a/src/tracking-information/index.js
+++ /dev/null
@@ -1,21 +0,0 @@
-export class TrackingInformation {
-  get shippingCompany() {
-    return this._shippingCompany;
-  }
-
-  set shippingCompany(arg) {
-    this._shippingCompany = arg;
-  }
-
-  get trackingNumber() {
-    return this._trackingNumber;
-  }
-
-  set trackingNumber(arg) {
-    this._trackingNumber = arg;
-  }
-
-  get display() {
-    return `${this.shippingCompany}: ${this.trackingNumber}`;
-  }
-}

diff --git a/src/tracking-information/index.test.js b/src/tracking-information/index.test.js
deleted file mode 100644
index 2ba2416..0000000
--- a/src/tracking-information/index.test.js
+++ /dev/null
@@ -1,12 +0,0 @@
-import { TrackingInformation } from '.';
-
-describe('TrackingInformation', () => {
-  it('should provide a readable display of its info', () => {
-    const trackingInfo = new TrackingInformation();
-
-    trackingInfo.shippingCompany = 'DHL';
-    trackingInfo.trackingNumber = '1234567890';
-
-    expect(trackingInfo.display).toEqual('DHL: 1234567890');
-  });
-});

And that's it!

Commit history

Below there's the commit history for the steps detailed above.

Commit SHA	Message
8e9ec1f	introduce delegate to shippingCompany at Shipment
df6db45	introduce delegate to trackingNumber at Shipment
79d6a0d	introduce getter for shippingCompany at Shipment
07dd9e3	introduce getter trackingNumber at Shipment
ffc8c87	inline trackingInformation.display into Shipment.trackingInfo
510c1cb	move shippingCompany field to Shipment
7c254eb	move trackingNumber field to Shipment
c5f7f7c	stop using TrackingInformation at Shipment
b38c7d5	delete TrackingInformation

For the full commit history for this project, check the Commit History tab.